Push Ctrl+` to load the console to test the following command. It will highlight the current selection with the coloring for scope "string" as defined in your color scheme. You can make it into a plugin command and then assign a key binding to that plugin command.

http://www.sublimetext.com/docs/2/api_reference.htmladd_regions(key, [regions], scope, <icon>, <flags>)The scope is used to source a color to draw the regions in, it should be the name of a scope, such as "comment" or "string". If the scope is empty, the regions won't be drawn.